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STRONG ANISOTROPY IN THE CUBIC FERRIMAGNET FeCr2 S4
R. van Stapele, J. van Wieringen, P. Bongers
To cite this version:
R. van Stapele, J. van Wieringen, P. Bongers. STRONG ANISOTROPY IN THE CUBIC FERRIMAGNET FeCr2 S4. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-53-C1-54.
�10.1051/jphyscol:1971111�. �jpa-00213925�
JOURNAL DE PHYSIQUE Colloque C I , supple'rnent au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1 - 53
STRONG ANISOTROPY IN THE CUBIC FERRIMAGNET FeCr, S4
R. P. van STAPELE, J. S. van WIERINGEN and P. F. BONGERS Philips Research Laboratories N. V. Philips' Gloeilampenfabrieken
Eindhoven-Netherlands
RksumB. - On a mesure l'aimantation des monocristaux de FeCrzS4 en fonction du champ magnetique dans les directions [loo], [I101 et [ I l l ] entre 4 OK et Tc = 195 OK ; [loo] est l'axe d'aimantation facile et la direction [Ill] est tres difficile. A 4 OK la rigidit6 magnetique dans la direction [loo] correspond a Ki = 3 x 106 erg. cm-3. Des spectres Mossbauer du 57Fe dans FeCrzS4 ont 6tk mesures ti 4 et 77 OK. A 4 OK il y a un dkdoublement quadrupolaire non axial.
Dans Cd0.9857Feo.ozCrzS4 2i 4 OK, au contraire, le dkdoublement quadrupolaire est axial. L'anisotropie magnktique et les dCdoublements quadrupolaires sont discutks partant d'un schema de niveaux d'knergie d'un ion Fez+ isole situk dans un site tetraedrique.
Abstract. - The magnetization of single crystals of FeCrzS4 has been measured between 4 OK and Tc = 195 OK with the field in the [100],, [110] and [Ill] directions. [loo] is the easy axis of magnetization, [Ill] is very hard. At 4 OK the st~ffness In the [loo] dlrectton corresponds to K1 = 3 x 106 erg.cm-3. 57Fe Mossbauer spectra of FeCrzS4 has been obtained at 77 and 4 OK. A non axial quadrupole splitting is observed at 4 OK. A uniaxial quadrupole splittinng is found in C ~ I - ~ F ~ S ; C ~ ~ S ~ (x = 0.02) at 4 OK. The magnetic anisotropy and the quadrupole splittings are discussed on the basis of a single ion energy level scheme of tetrahedrally coordinated Fezf.
I. Introduction. - FeCrzS4 crystallizes in the spinel structure with Fez+ on the tetrahedral sites and Cr3+
on the octahedral sites. In contradistinction to FeCrz04 which is tetragonal below 11 1 OK [I], FeCrzS, is cubic down to 4 OK as measured by neutron diffraction [2]. The spin ordering was found to be of the simple N6el type. The compound is interesting since 1) it exhibits a quadrupole splitting in the 57Fe Mossbauer spectrum at T < Tc in spite of the local cubic symmetry and 2) there are indications of a strong magneto-crystalline anisotropy below 80 OK.
Both facts were attributed by Eibschutz et al. [3] to the splitting of the 5E ground state of Fez+ on tetra- hedral sites by spin-orbit interaction and Cr3 + -Fez +
exchange interaction. These authors concluded from a calculation within the simple crystal field model for tetrahedrally coordinated Fez+ that the cubic axes are preferred directions for the magnetization and that the splitting of the ground state gives rise to an electric field gradient on the nucleus, axially symmetric around the direction of the magnetization along a cube axis.
They were able to interpret the Mossbauer spectrum measured down to 22 OK in agreement with this model.
However, Hoy et al. [4] found in their Mossbauer spectra of FeCrzS4 taken between 61 and 168 OK deviations from axial symmetry.
The present paper will present new experimental results on the Mossbauer spectrum of FeCr2S4 at 4 OK, on the magnetic anisotropy of this compound and on the low temperature Mossbauer spectrum of
Cd0.98Fe0.0ZCr2S4
.
The last compound is a very useful one since it enables one to observe the behaviour of single Fez+ ions in the exchange field arising from the Cr moments which are themselves coupled ferromagnetically below
Tc = 86°K.
11. Experiments. - FeCr,S4 was prepared by heat- ing an intimate mixture of CrzS,, FeS1-, and S at 700 0C in an evacuated silica ampoule. After ball milling the firing was repeated at 800 OC. X-ray powder diagrams showed the samples to be single phase spinel.
The single crystals of FeCrzS4 were grown by a vapour-liquid-solid transport reaction [5]. FeCrzS,
powder was maintained at 870 OC at one end of a closed silica tube containing % 20 Torr C12 gas. At the cold end of the tube (8500C) FeCrzS4 octahedra grow inside liquid droplets (mainly FeC1,) in sizes of 1 till 4 mm. In some of the experiments plate shaped FeCr,S4 crystals and Cr2S3 crystals have been found at the position of the feedtng material. Chemical ana- lysis of different batches of crystals proved the Cr to Fe ratio to vary from,:! to 1.9 indicating that in the crystal used for the magnetic measurements less than 2
%
of the Cr may be replaced by Fe.Polycrystalline CdCrzS4 in which 2
%
of the Cd has been replaced by 57Fe was prepared in a similar way as described above for FeCrzS4. 57Fe was obtained by reducing enriched 57Fez03 containing 70%
57Fe.The magnetization has been measured with the magnetic field parallel to [100], [I101 and [ I l l ] on a single crystal slab of FeCrzS4 cut along the (110) faces between 4 OK and 200 OK and fields of 3, 6, 12 and 18 kOe. With HI/ [I001 saturation of the magnetization is reached in these fields. With H parallel to [ I l l ] and [I101 no saturation is obtained even in the highest field (18 kOe) for T < 60 OK. At temperatures below 60 OK o,,, < o,,, < oleo for the same temperature (see Fig. 1). These results indicate [loo] to be the
FIG. 1. - Temperature dependence of the magnetization of FeCrzS4 with the field H parallel to the [I001 and [ I l l ] directions.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971111
C 1 - 5 4 R. P. VAN STAPELE, J. S. VAN WIERINGEN AND P. F. BONGERS direction of easy magnetization in the whole tempera-
ture range, [I 111 being the hard direction.
At 40K o, = 36 gauss cm3/g corresponding to nB = 1.86 per mol FeCr2S4. For a N6el type spin ordering and assuming a moment of 3 pB for the Cr3+ ions one finds a moment for the tetrahedral Fe2' of 4.14 pB considerably lower than 4.4 pB [6]
deduced from powder measurements. Gibart et al. [7]
observed a saturation moment nB = 1.79 pB on a powdered crystal of FeCr2S4. Their a
-
T curve is very similar to our curve with HI/ [loo]. Presumably the crystallites have oriented themselves in the magne- tic field.Mossbauer spectra of FeCr2S, and Cd0.98Fe~.~2Cr2S4
have been taken at 4 OK and 77 OK. The spectra were described by the usual nuclear spin hamiltonian gn P n H h f I z
+
A[(3 I; - I(I+
1))+
v(I2- 1,211
The orientation of the axes of the electric field gradient are given by W and q, the angles between the z-axis and the C and axis respectively. Although the 4 level splittings (3 in the upper quartet and 1 in the lower doublet) are insufficient to determine the 5 pertinent parameters H, A, y,
V,
cp unambi- guously, fairly restricted intervals for the parameter values can be derived from the experimental spectrum : H = 186 kg, - 0.441 < A <-
0.440 mmls, 870 <W
< 90°, 0.21 < y < 0.25, 0 < q < 170. This shows that apart from the qualitative deviations from the Eibschutz single-ion theory there are also quanti- tative deviations because H is an order of magnitude larger. In order to see, if this is a cooperative effect the case of a single ferrous ion in a similar exchange field was studied by replacing 2%
of Cd in CdCr2S4 by 5 7 ~ e . The Mossbauer spectra are quite differentFIG. 2. - Mbssbauer spectra at 40K of FeCrzS4 (top) Cdo.9857Feo.02Cr2S4 (bottom). The source used is 57C0 in
palladium.
FRANCOMBE (M. H.) J. Phys. Ckem. Solids, 1957,3,37.
BROQUETAS COLOMINAS (C.), BALLESTRACHI (R.) and ROULT (G.), J. Physique, 1964, 25, 256.
SHIRANE (G.), COX (D. E.), PICKART (S. J.), J. Appl.
Pkys., 1964, 35, 954.
EIBSCHTEZ (M.), SHTRIKMAN (S.) and TENENBAUM (Y.), Phys. Lett., 1967, 24A, 563.
HOY (G. R.) and SINGH (J.), Phys. Rev., 1968,172,514.
from those in FeCr2S4 (Fig. 2) and can be readily explained in terms of an axial electric field gradient parallel to the magnetization with A N 0.01 mm/s at 77 OK and
+
0.47 mm/s at 4 OK, and a Zeeman field of 130 kG at 77 OK and 51 kG at 4 OK.111. Discussion. - From the magnetization data the usual cubic anisotropy constants Kl and K2 can be evaluated. At 40K the constant Kl equals 2.0 cm-l per Fe2+, corresponding to 3 x lo6 erg. ~ m - ~ . Such a large anisotropy cannot be attributed to the Cr3+ ions or to a small number of Fe2+ on octahedral sites. It must originate from the tetrahedrally coordinated Fe2+. Kl decreases with increasing temperature to a value of 1.0 cm-l at 45OK. The constant K2 was found to be positive and was estimated to be of the same order of magnitude.
In a simple crystal field model for the splitting of the 5E ground state of ~ e on a tetrahedral site by ~ + the combined action of the exchange field from the Cr moments and spin orbit interaction with the excited 5T2 crystal field level one finds that
Kl = 3d2(4 - 4 <
s:> + < $>)I8 k~
at temperatures such that kT
+
6. The constant 6 equals 6 12/A, where I is the strength of the spin orbit interaction and A , the cubic crystal field splitting.At 45 OK K , = 1 cm-l per Fe2+, which corres- ponds to 6 = 4.6 cm-l. This value of 6 is close to 5 cm-', the value obtained by Eibschutz et al. [3] in the temperature range of 22 to 77 OK. However, at 4 OK we find 6 = 2.3 cm-l from a similar analysis of the magnetization data at temperatures where kT -- 6.
The Mossbauer data on Feo,02Cdo,,8Cr,S4 points to a simpler behaviour of a single Fe2+ in the exchange field of the ferromagnetic Cr lattice. Using the for- mulae given by Eibschiitz et al. for the temperature dependence of the Zeeman field and the quadrupole splitting we obtain 6 = 4.6 cm-', close to the value found in FeCr2S4 at 45 OK.
The small value of 6 deduced from the magnetic anisotropy of FeCr2S4 at 4 OK is correlated with the deviation from axial symmetry in the Mossbauer spec- trum, which points to a local symmetry lower than cubic at the tetrahedral sites. Consequently the orbital doublet ground state of Fe2' is expected to be split by the low symmetry component of the crystal field.
Even a small splitting of the order of 10 cm-I is expected to reduce the magnetic anisotropy appre- ciably.
Acknowledgement.
-
The authors wish to thank J. F. Fast, A. B. Voermans and J. G. Rensen for assistance with the preparation and measurements.Eindhoven, August 1970.
[5] GIBART (P.) and BEWUEN-DEMEAUX (A.), Acad. Sci., Paris, 1969, 268, s6rie C, 816.
SHICK (L. K.) and VON NEIDA (A. R.), J. Cryst.
Growth, 1969, 5, 313.
[6] LOTGERING (F. K.), VAN STAPELE (R. P.), VAN DER
STEEN (G. H. A. M.) and VAN WIERINGEN (J. S.), J. Phys. Chem. Solids, 1969, 30, 799.
[7] GIBART (P.), DORMANN (J. L.) and PELLEGRIN (Y.), Phys. Stat. Sol., 1969, 36, 187.
